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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Stochastic environmental research and risk assessment 3 (1989), S. 17-29 
    ISSN: 1436-3259
    Keywords: Stochastic optimization ; linear programming ; simplex method ; Karmarkar's method
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Energy, Environment Protection, Nuclear Power Engineering , Geography , Geosciences
    Notes: Abstract Optimization of multi-reservoir systems operations is typically a very large scale optimization problem. The following are the three types of optimization problems solved using linear programming (LP): (i) deterministic optimization for multiple periods involving fine stage intervals, for example, from an hour to a week (ii) implicit stochastic optimization using multiple years of inflow data, and (iii) explicit stochastic optimization using probability distributions of inflow data. Until recently, the revised simplex method has been the most efficient solution method available for solving large scale LP problems. In this paper, we show that an implementation of the Karmarkar's interior-point LP algorithm with a newly developed stopping criterion solves optimization problems of large multi-reservoir operations more efficiently than the simplex method. For example, using a Micro VAX II minicomputer, a 40 year, monthly stage, two-reservoir system optimization problem is solved 7.8 times faster than the advanced simplex code in MINOS 5.0. The advantage of this method is expected to be greater as the size of the problem grows from two reservoirs to multiples of reservoirs. This paper presents the details of the implementation and testing and in addition, some other features of the Karmarkar's algorithm which makes it a valuable optimization tool are illuminated.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of heuristics 3 (1997), S. 225-243 
    ISSN: 1572-9397
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Performance-driven physical layout design is becoming increasingly important for both high speed integrated circuits and printed circuit boards. This paper studies the problem of assigning wire segments into two layers so as to minimize the number of vias, while taking into account performance constraints such as layer preference and circuit timing. We show that using the Elmore delay model, three timing problems in synchronous digital circuits—the long path problem, the short path problem and the time skew problem—can be formulated as a set of linear inequalities. We use the model of signed hypergraph to represent two-layer routings and formulate the performance-driven optimum layer assignment problem as the path-constrained maximum balance problem in a signed hypergraph. Two solution methods are developed and implemented. First, an integer linear programming formulation is derived for finding exact solutions. Second, a local-search heuristic for hypergraph partitioning is extended to cope with path-inequality constraints. Experimental results on a set of layer-assignment benchmarks demonstrated that the path-constrained local-search heuristic achieves optimum or near-optimum solutions with several orders of magnitude faster than the integer linear programming approach.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-2277
    Keywords: Key words Orthotopic liver transplantation ; Hepatitis B virus ; Hepatitis C virus ; Long-term liver transplant survival ; Risk factors
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Orthotopic liver transplantation (OLT) is used as a definitive treatment for end-stage liver disease and prolonged posttransplant survival has already been reported. The incidence of late mortality and graft morbidity is, however, not well defined and the role of primary viral disease in the long-term follow-up results is not clear. Data of posttransplant follow-up in 213 patients, 156 adults and 57 children, who survived at least 1 year were reviewed in order to define causes of graft dysfunction, graft loss and death. In 98 patients, 103 persistent graft dysfunctions were found. Thirty-four grafts were later lost [28 deaths and 6 successful retransplantations (re-OLT)]. The results were reviewed grouping patients according to their age and viral hepatitis status at the time of the transplantation. HBV-positive patients (51) showed 4 re-OLT (1 HBV), 3 liver-related deaths (2 HBV), 24 graft dysfunctions (8 HBV, 5 HCV), and 85.2 % 6-year survival (based on 100 % survival at 1 year). HCV-positive adults (28) showed 1 re-OLT, 3 HCV-related deaths, 24 graft dysfunctions (19 HCV), and 68.8 % 6-year survival. HBV–HCV-positive patients (14) showed no graft loss and death, 10 graft dysfunctions (7 HCV, 1 HBV, 2 HBV–HCV), and 81.8 % 6-year survival. HBV–HCV-negative adults (63) showed 3 non-hepatitis-related re-OLT, 5 liver-related deaths (2 HCV), 24 graft dysfunctions (6 HCV, 2 HBV), and 83.1 % 6-year survival. HBV–HCV-negative children (49) showed no re-OLT, 1 HCV-related death, 14 graft dysfunctions (3 HCV), and 92.6 % 6-year survival. HCV-positive children (8) showed 1 HCV-related re-OLT, 2 HCV-related deaths, 4 graft dysfunctions (3 HCV), and 81.3 % 6-year survival. The main cause of graft dysfunction was hepatitis (45 HCV and 13 HBV), followed by technical complications (21), rejection (16), recurrent alcoholism (3), HIV infection (1), and unknown causes (4). In this long-term posttransplant follow-up series, viral hepatitis led to graft dysfunction in 58/103 (56.3 %) cases, late graft failure was viral hepatitis-related in 11/20 (55 %) cases, and, as a total, HCV infection was present in 45/58 (77.5 %) cases of viral hepatitis-related graft damage. Looking at the timing of hepatitis-related graft failure, in 70 % of cases death occurred after the 5th post-transplant year. In our experience, the occurrence of hepatitis, particularly HCV induced, was common and led to abnormal graft function, but the 6-year posttransplant survival (based on 100 % survival at 1 year) in patients surviving for at least 1 year did not differ on the basis of the pretransplant viral hepatitis status. This finding may be consistent with the slow progression of the viral damage and longer follow-up results remain to be established. Nevertheless, data from the present study suggest that in long-term liver transplant survivors, the risk of deteriorating liver damage and eventual failure after 5 years remains only in those patients experiencing a viral hepatitis infection.
    Type of Medium: Electronic Resource
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